A quantitative analysis of the degree and extent of coronary arterial narrowing by atherosclerotic plaques in the entire lengths of each of the 4 major coronary arteries in 27 necropsy patients with acute transmural myocardial infarction is described at necropsy for the first time. Of the 1403 five-mm segments examined in the 27 study patients, 484 (34%) were 76-100% narrowed in cross-sectional area by atherosclerotic plaques (controls equals 3%); 528 (38%) were 51-75% narrowed (controls equals 25%); 319 (23%) were 26-50% (controls equals 44%) and only 72 segments (5%) were greater than 25% narrowed (controls equals 28%).